The problem of blood flow control through an arterio-venous fistula or a veno-venous fistula is described, inter alia, in the following publications:
1. "Pressure relations at site of an arterio-venous fistula": Emile Holman and Gerard Taylor, Angiology 3, 415-430, 1952. PA1 2. "Portacaval H Graft : Relationships of shunt diameter, portal flow patterns and encephalopathy": James Sarfeh et al, Ann. Surg. Vol. 197 No. 4 422-426. PA1 (i) a valve body defining a valve chamber PA1 (ii) an inlet to and an outlet from the chamber, PA1 (iii) a valve seat member having: PA1 (iv) a valve member overlying and normally closing the annular valve seat, said valve member being coupled to the plunger so that when the valve body is distorted by the application of an external compressive force at or adjacent to the inlet, the valve seat member is so distorted that the plunger moves towards the outlet whereby the valve member is moved away from the valve seat so as to place the outlet in communication with the inlet. PA1 (i) a valve body defining a valve chamber, PA1 (ii) an inlet to and an outlet from the valve chamber, PA1 (iii) a valve seat member disposed across the valve chamber and having a bore in communication with the inlet and selectively in communication with the outlet, PA1 (iv) a perforated support member positioned in the valve chamber between the outlet and the valve seat member and having a valve plunger extending thereform adapted to engage and close the bore of the valve seat member, the arrangement being such that when the valve body is distorted by the application of an external compressive force between the perforated support member and the valve seat member, the valve seat moves away from the plunger towards the inlet whereby the outlet is placed in communication with the inlet.
As stated by Holman and Taylor, the effects of a fistula depend in large measure upon its size, but even more significantly, upon the relationship of its size to the calibre of the vessels in which the fistula lies. Bearing directly upon this relationship is the concept that a vessel at any given point in the arterial tree has an arterial end pressure sufficient to overcome the peripheral resistance distal to it, the one being commensurate with the other. The larger the vessel, therefore, the greater will be the peripheral resistance of the capillary bed supplied by it, and consequently the greater must be the arterial end pressure to overcome it.
Also, the larger the vessels between which a fistula lies, the greater will be the difference between the low pressure of the central venous bed and the high peripheral resistance of the capillary bed distal to the fistula, thus increasing the tendency for blood to avoid the capillary bed and to flow into the central venous bed.
Moreover, the larger the artery in which the fistula lies, the higher will be the artery end pressure directing blood through the fistula into the large central reservoir of low venous pressure with a corresponding increase in velocity of blood through the fistula. Given a uniform size of fistula, the nearer to the heart this fistula lies in the main arterial tree, the greater will be the volume of blood pouring through it.
Thus, the work of Holman and Taylor was, in essence, concerned with the location and size of a fistula rather than upon the control of flow of blood through the fistula.
The work of Sarfeh et al was concerned with the use of portacaval H-grafts of different diameters and not with the control of blood flow through the grafts.
Although neither of the abovementioned publications refers to any device for controlling the flow of blood through a fistula, an implantable device for restricting the flow of blood through a major blood vessel such as an artery is disclosed in U.S. Pat. No. 3,730,186 of Edmunds et al.
The Edmunds device consists of an inflatable, flexible annulus, generally circular in shape but not a closed circle, which has a non-distensible outer wall so that upon inflation all distention or expansion is inward.
The Edmunds device is placed around an artery or other blood vessel and the ring is then closed by suturing together pre-formed tabs attached to the annulus, or by suturing together the ends of an overlaping tape to hold the vessel firmly.
Upon inflation, inward distention of the inflatable annulus constricts the vessel and flow of blood therethrough is accordingly restricted. Inflation and deflation are effected through a self-sealing hollow bulb and a non-distensible tube connecting the bulb to the interior of the inflatable annulus.
Although U.S. Pat. No. 3,730,186 does disclose an implantable adjustable extravascular occluding band adapted to restrict the flow of blood, the only application described in the specification is concerned with the control of blood flow through an artery and does not relate to the diversion of blood flow from arteries between veins and venous sinks.
In general terms, blood flows from an artery through an organ (e.g. liver) which may be considered as an venous sink and then back to a vein. When the normal blood flow system ceases to operate correctly, various kinds of vascular problems may arise.
For example, in recent years, it has become increasingly clear that very frequently erectile impotence is associated with penile vascular problems, although they may not be the only contributive factors to the symptom which is often multi-determined.
For the sake of brevity, the invention will be described in relation to vasculogenic impotence but it is to be understood that the invention is not limited thereto as its principles may be applied to any situation in which the flow of blood is to be controlled.
Penile arterial inflow is through three pairs of arteries (dorsal artery, deep artery and bulbourethral artery) which are branches of the internal pudendal artery. There is great variation in their branching, the position at which they pierce the tunica albuginea and how they communicate. The glands receives its main arterial supply from the dorsal artery, the cavernous bodies from the paired deep arteries. There are arteriovenous shunts at many levels of arterial branching-outside as well as inside the tunica albuginea.
The von Ebner pads protrude not only in to the small penile arteries as originally described, but also occur in almost all parts of the arterial tree supplying the erectile tissue as far back as the penile artery after it passes through the urogenital diaphragm. Their exact function is unknown, although they may be involved in flow regulation. The venous drainage mainly takes place through the systems of the deep dorsal vein and the deep central veins.
There are theories as to the vascular mechanisms of penile erection, but the exact mechanism is obscure as no theory can encompass all the facts and clinical observations. Both inflow and outflow (venous drainage) regulatory mechanisms appear to be involved simultaneously.
The prior art contains various proposals for overcoming erectile impotence. For example the Jonas penile prothetis consists of silver wires embedded in a silicone tube. The Finney penile implant consists of a hinged flexirod device made of clear silicone. Other devices include the Scott prosthesis which is a totally implantable device using inflatable silastic cylinders placed inside each corpus cavernosum and connected by silastic tubing to a pumping mechanism implanted in the scrotal pouch, the fluid for inflation being provided by a reservoir implanted behind the rectus muscle.
In 1975 Small and Carrion reported the development of a silastic prosthesis which consisted of two semi-rigid moulded silicone rods implanted side by side in the matrix of each corpus cavernosum.
None of the aforementioned prothesis has been particularly successful in overcoming erectile impotence as not one overcomes the problem of impaired circulation.
Other vascular disorders are well known and their correction along with that for vasculogenic impotence necessitates the establishment of an alternative vascular path.
Other implantable devices for temporarily controlling the flow of blood or other body fluids are known.
A typical example of such an implantable device is the so-called urinary incontinence prosthesis which has an inflatable urethral occluder that is connected by tubing to a reservoir bulb. The bulb has a squeeze deformable valve operative to allow flow of fluid from the bulb to the occluder and, when squeezed, to allow flow of fluid from the occluder to the reservoir bulb.
Usually, the reservoir bulb and the deformable valve are located in the scrotum so that a squeeze action can be easily applied to the bulb to force fluid through the valve to the occluder. When the occluder is to be relaxed, the valve is squeezed whereupon it opens to allow the fluid to flow back into the reservoir bulb.
The reservoir and valve are so located in the body as to be readily manipulated by the fingers. There is a need for a normally closed valve that is able to pass fluid from the reservoir when the reservoir is squeezed (or otherwise deformed) by pressure applied through the skin and which can be easily opened to allow the fluid to flow back into the reservoir.
One such valve which is disclosed in U.S. Pat. No. 3,758,073 consists of a valve body having an internal flow cavity bounded by a peripheral wall. An inlet port which could be connected to the occluder or balloon of the above described devices communicates with the flow cavity through an opposite end wall. A flow control member has a central aperture in a base that bears against the opposite end wall, the aperture being aligned with the outlet port.
A valve seat formed as an annular ring about the aperture in the flow control member is covered by a flexible diaphragm that is supported by the periphery of the flow control member. The central portion of the diaphragm is imperforate so that when the diaphragm is against the valve seat, the valve is normally closed. A plurality of perforations formed in the diaphragm outside the central imperforate region provide fluid communication across the diaphragm.
When it is desired to open the valve, the body is squeezed to deform the diaphragm which lifts from the valve seat so that fluid can flow through the perforations and then through the outlet to the reservoir so as to depressurize the occluder or balloon. The flexibility of the diaphragm is such that when flow is to be reversed, pressure on the reservoir will lift the central portion of the diaphragm from the valve seat.
A disadvantage of the above kind of valve is that the opening pressure is not positively applied to the valve diaphragm, but, rather it is the deformation of the valve body which leads to wrinkling of the diaphragm and it is the degree and manner of wrinkling which leads to the displacement of the valve diaphragm from the valve seat.